In today's cellular systems, public cellular systems typically coexist with autonomous systems. An autonomous system is a private or residential cellular system that shares frequencies with the public cellular system. An autonomous system is typically located within a public cellular provider's radio coverage area, but has limited geographical reach, such as a building or campus. The autonomous system typically shares radio spectrum with the public cellular system so as to allow compatibility with existing mobile cellular telephones. However, the microsystem virtually always has the responsibility to avoid interference with the frequencies of the public cellular system.
One such cellular system is described in Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA) standards document ANSI/TIA/EIA-136, “TDMA Cellular PCS,” Rev. B, Mar. 1, 2000, which is hereby incorporated by reference, which is a multi-part standard that defines the requirements for a PCS/cellular system using Time Division Multiple Access (TDMA) technology. This system uses Digital Control Channels (DCCHs) that communicate between the cellular network Base Station/Mobile Switching Center/Interworking functions (BMIs) and the mobile station (MS) of the cellular subscriber. Information exchanged on a DCCH enables the subscriber to gain access to the cellular system and, among other things, place and receive cellular calls. Once calls are established, voice and data traffic is carried on Digital Traffic Channels (DTCs).
The geographic service area covered by a public cellular provider's cellular system is divided into a number of generally contiguous cells, each cell having at least one DCCH and a number of DTC channels based on the desired traffic capacity for the cell. In an ANSI/TIA/EIA-136 compliant system, communications between the cell site base station and mobile stations in the base station coverage area take place over several defined broad frequency ranges. Each broad frequency range is divided into a set of defined frequencies, or physical channels. Each physical channel is divided into a plurality of TDMA time slots. Communications between the base station and the MS occurs as short digital bursts formatted to fit within a time slot, with each time slot tagged with identifier information. Messages exchanged between the base station and the MS typically span several contiguous time slots. There are two main types of communications between the base station and the MS: control information and digital traffic information. Message streams on a physical channel comprising messages associated with control functions are collectively referred to as the DCCH logical channel, and message streams comprising messages associated with digital traffic are collectively referred to as the Digital Traffic Channel (DTC) logical channel. Each main set of message types can be further divided into sets of message subtypes, with each message subtype set associated with a specific control or traffic function. Message streams comprising messages of one of the subtypes are referred to as logical subchannels, or simply channels, of the logical channels.
In the context of an autonomous system, the DCCH operating frequencies in use by the public cellular providers in the area of the autonomous system are referred to as Public Service Profiles (PSPs), and the DCCH operating frequencies in use by the autonomous systems are referred to as Private Operating Frequencies (POFs).
In a cellular system, a MS must always be monitoring, or “camped,” on a DCCH in order to exchange the control information necessary to, among other things, place and receive phone calls. When a MS is powered on or otherwise initially appears in a cellular network, the MS must identify a suitable DCCH on which to camp. Section 3 of part 136-910 of ANSI/TIA/EIA-136 describes a reference model process that a MS may use to select a control channel. In this model, a MS first checks for service on the last used DCCH. If this DCCH is not found, the MS checks the POFs of autonomous systems (that the MS has stored in its memory) on which the MS has successfully registered in previous attempts. If a DCCH is not found, any methodology that a MS designer devises may be used to locate a DCCH before commencing a search for a DCCH by examining Analog Control Channels (ACCs) and DTCs for pointers to DCCHs, or by a frequency by frequency search of digital channels on the current band.
If a DCCH is found, it is examined for suitability from a signal strength perspective and from a service aspects perspective. If the DCCH fails the signal strength criteria or the service aspects criteria, the MS continues searching for a suitable DCCH. If the signal strength criteria and the service aspects criteria are met, the MS camps on this DCCH.
After the MS identifies and camps on a suitable DCCH, one of the first things it is required to do is make an initial reading of a full cycle of the Fast Broadcast Channel (F-BCCH) and the Extended Broadcast Channel (E-BCCH) subchannels of the DCCH and update any indicated parameters in the MS. The MS continuously monitors the F-BCCH and E-BCCH for updated data. The information broadcast on the F-BCCH and E-BCCH provides, among other things, a Neighbor List (NL) of nearby DCCH channels, including, in the case of an autonomous system, PSPs and POFs. After MS parameters have been updated with information from the F-BCCH and E-BCCH, the MS begins monitoring the signal strength and quality of the serving DCCH and of all DCCHs in the NL.
After a MS has camped on a DCCH, there are conditions that will trigger the MS to reselect a new DCCH on which to camp. These Reselection Trigger Conditions (RTC) are listed in Section 4.3.4.1 of part 123 of ANSI/TIA/EIA-136. The RTC are related to conditions such as poor signal strength or poor signal quality of the current DCCH, monitoring of signal strength and quality of the current DCCH and all DCCHs in the Neighbor List identifying a better DCCH than the current one, the current DCCH is barring the MS from camping, the MS receiving a message explicitly directing it to another DCCH, and the MS or MS user deciding to acquire service on a DCCH supporting a system of higher priority relative to the current DCCH, such as a private or residential system. In these conditions, the NL entries acquired on the current DCCH, POFs and/or a DCCH identified when an MS executes a band scan for DCCHs as a result of Non-Public Mode Search, serve as the candidate DCCHs for reselection.
The Control Channel Reselection procedure is described in Section 4.3 of part 123 of ANSI/TIA/EIA-136. This procedure comprises two sub-procedures. The Control Channel Locking procedure identifies suitable candidate DCCHs for reselection based on signal strength and quality. The Reselection Criteria procedure chooses a DCCH from the candidate DCCHs based on the RTC that caused the Control Channel Reselection procedure to be invoked.
In the context of a public cellular system, i.e., one in which there are no autonomous systems in the operating area of the MS, the Control Channel Reselection procedure is typically straight forward. The candidate DCCHs for reselection are the NL entries received by the MS in the initial reading of the E-BCCH after camping on the DCCH. These entries will generally be the DCCHs of the cells or sectors closest to the serving cell site. When the Control Channel Reselection procedure is invoked, the MS first checks for service on the last used DCCH, and then checks for service on the DCCHs of these closest cells or sectors. The MS quickly locates and camps on the last used DCCH if, for example, the MS temporarily lost signal, or on a DCCH geographically near to the last used one if, for example, the MS has moved to an area where the new DCCH signal is stronger than that of the last used DCCH.
When an MS is used in the context of an autonomous system, additional procedures are used in the Control Channel Reselection process. Because there may be a large number of autonomous systems in a public cellular service provider's area, and because the frequencies in use for each autonomous system are usually dynamic, the public provider does not include the DCCHs, or POFs, of the autonomous systems in the NL entries that are transmitted to the MS. Therefore, the normal automatic reselection procedures which are based solely on NL entries must be supplemented when a MS is searching for autonomous systems. When the channel reselection procedure is triggered on a MS camped on a public system DCCH, the MS must consider the POFs of the autonomous system as candidates for reselection.
Section 4.20 of part 123 of ANSI/TIA/EIA-136 describes the additional procedures that must be executed by a MS when control channel reselection procedures are triggered in an autonomous system. A mobile station will store two sets of frequencies for each autonomous system's Private System Identification (PSID) or Residential System Identification (RSID) that it retains in memory. The first set of frequencies correspond to DCCHs that have been assigned to the public cellular network BMIs in the general vicinity of the autonomous system. These frequencies, along with associated identification data, such as the public provider's System Identification (SID) and the channel's Digital Verification Color Code (DVCC), are referred to as the Public Service Profiles (PSPs) of the public system(s). The second set of frequencies represent candidate DCCH frequencies of the autonomous system, and are termed the Private Operating Frequencies (POFs). The standard calls for a mobile station to allow for the storage of a minimum of eight PSPs and eight POFs per autonomous system PSID or RSID.
Each time a mobile station camps on a DCCH, the frequency and DVCC of each stored DCCH PSP are compared to the frequency and DVCC of the current DCCH. If both the frequency and DVCC of any of the stored PSPs match the frequency and DVCC of its current DCCH, then a candidate autonomous system is considered as identified and the mobile station proceeds to examine the supplementary PSP information as follows.
If the SID associated with the PSP under consideration corresponds to the PSID/RSID of the candidate autonomous system, the mobile station declares a PSP match.
In other words, the MS has camped on a DCCH of the public system that is identified in the PSP/POF tables of the MS as being in the vicinity of a known DCCH of an autonomous system.
If the SID associated with the PSP under consideration does not corresponds to the PSID/RSID of the candidate autonomous system, the mobile station declares a PSP mismatch for the PSP under consideration
Each time a mobile station tunes to the strongest or second strongest dedicated Analog Control Channel (ACC) while performing an Initialization task, the frequency, SID and Digital Color Code (DCC) of each stored ACC PSP are compared to the frequency, SID and DCC of this ACC. If the frequency, SID and DCC of any of the stored PSPs match with the frequency, SID and DCC of the strongest or second strongest dedicated ACC, the mobile station declares a PSP match. Otherwise, the mobile station declares a PSP mismatch for the PSP under consideration.
When a PSP match is declared while on a DCCH, the mobile station adds the POFs of the associated autonomous system to the list of channels identified as requiring signal strength measurements. The mobile station then, after an appropriate delay as required for channel measurement purposes, declares a Priority System Condition and uses the POFs as the list of reselection candidates. The mobile station also determines the MS-ACC-PWR, RSS-ACC-MIN, SS-SUFF and DELAY for the POFs prior to invoking or while executing the Control Channel Reselection procedure. When a PSP match is declared while on an ACC, the mobile station may determine that a DCCH is the preferred service provider, and enter the Control Channel Scanning and Locking State, using the associated POFs as candidates.
A mobile station will allow for manual initialization of PSPs and POFs, and will also allow for automatic initialization of PSPs and POFs upon initial selection of an autonomous system as follows:
Whenever a mobile station camps on a DCCH supporting a PSID or RSID that matches a PSID/RSID stored in its memory, and the Public bit of a Network Type indicator is set to zero, the mobile station updates the PSPs and POFs stored for the corresponding PSID/RSID;
To update the PSPs, the mobile station will store the first eight neighbor list entries received within a Neighbor Cell message or a Neighbor Cell (Multi Hyperband) message, that have a CELLTYPE of NON-PREFERRED. A CELLTYPE of NON-PREFERRED refers to a public provider's system. The mobile station first examines the Neighbor Cell List (TDMA) information element in an attempt to find eight neighbors having a CELLTYPE of NON-PREFERRED. If eight neighbors are not found in the Neighbor Cell List (TDMA) information element, the mobile station proceeds to examine the Neighbor Cell List (Other Hyperband) and then the Neighbor Cell List (Analog) information elements for additional neighbors;
To update the POFs, the mobile station stores the first eight neighbor list entries received within the Neighbor Cell message or the Neighbor Cell (Multi Hyperband) message, that have a CELLTYPE of PREFERRED or REGULAR. A CELLTYPE of PREFERRED or REGULAR refers to an autonomous system. The mobile station only examines the Neighbor Cell List (TDMA) information element in an attempt to find eight neighbors having a CELLTYPE of PREFERRED or REGULAR;
Whenever the mobile station stores a new set of PSPs or POFs for a given autonomous system PSID, it deletes the previous PSPs or POFs for that autonomous system PSID.
As mentioned above, an autonomous system base station shares frequencies with the cellular network of the public providers. The public system providers have mandated, in supplementary and proprietary specifications, that the private systems must react quickly to frequency changes in the public system to guarantee that the private systems are not transmitting on a frequency in use by that public system. This requires that the autonomous system constantly monitor the in-use frequencies of the public system, and then retune its own transceivers to channels that will not adversely affect RF performance in the public system. The autonomous system base station typically uses currently unused DTC channels of the public system for its own control and traffic channels. Since the demands of the public system are dynamic, the public system may at any time begin transmitting on a frequency that the autonomous system is using. When this occurs, the autonomous system must quickly abandon that frequency and move to a new frequency that is currently unused by the public system.
In an ANSI/TIA/EIA-136 compliant cellular network, a problem occurs in an autonomous system when a base station must move from a current DCCH to a new channel for use as a DCCH. The change in DCCH frequency forces all mobile stations that are camped on the current DCCH channel to execute a Control Channel Reselection procedure when radio communications to the autonomous system are lost on the current DCCH channel. As a result of the Control Channel Reselection procedure being executed, the MS will not automatically reacquire service (i.e., “camp”) on the autonomous system after the DCCH change is complete, but will instead acquire service on a nearby public system, if possible.
From a public cellular provider perspective, DCCHs for a given cell change infrequently. Thus, the NL entries that are transmitted on the E-BCCH channel and downloaded by the MS reflect the current state of a very stable service-area-wide control channel topology. The control channel selection and reselection procedures are straight-forward for public systems, with the new control channel selection typically being a geographically contiguous neighboring public cell.
In a typical in-building autonomous system, coverage for the building is provided by one or more low power cells with each cell engineered to provide coverage to a specific zone of the building. The cell transmissions typically do not extend outside the shell of the building for any significant distance, although this depends on the materials of the building shell at any given point of the shell. Since the number of users of an autonomous system is comparatively low, an autonomous system cell typically transmits on a single DCCH. In a campus-wide autonomous system that may provide coverage to several buildings, open areas between buildings are typically outside the coverage area of the autonomous system so as to minimize interference with the public cellular systems in the area. The PSP/POF list transmitted by a cell of the autonomous system just described would typically include the DCCHs of all the nearby cells located in the building, plus the DCCHs of the cells that provide coverage of the entryways of other buildings of the campus-wide system. However, even though a campus-wide autonomous system may transmit on several cells, a MS is typically, at any given time, in the coverage area of a single DCCH.
When the autonomous system cell is forced to move from a current DCCH to a new DCCH frequency in order to avoid interference with a public system, DCCH transmissions on the current frequency abruptly end, and the mobile stations that are camped on the current DCCH must execute a Control Channel Reselection procedure. Since the POF entry in the NL referencing the current DCCH is no longer valid, and the MS is typically not in coverage areas of other cells of the autonomous system, the MS will locate and camp on a DCCH of one of the PSP entries of the public system in whose coverage area the desired autonomous system is located.
This reselection to a DCCH of the public system occurs automatically and unknown to the user of the autonomous system, with no urgent MS indication, such as an audible or vibratory alarm. This is very undesirable for the user. The user assumes the MS is registered with the desired autonomous system, and will begin to miss calls directed to it through the autonomous system.
Typically, the way a user becomes aware that the MS is no longer camped on the autonomous system is to notice that the system ID shown on the MS display screen is that of the public system and not that of the autonomous system. It may take some time before the user notices that a reselection to the public system has taken place.
One possible solution to this problem of automatic reselection to the public system without urgent notification is to constantly monitor the display screen of the MS. When there is indication that the MS has reselected to the public system, the user can invoke the New PSID/RSID Search procedure by navigating the user menus of the MS and selecting the proper option. This procedure, as described in Section 4.18.1. of part 123 of ANSI/TIA/EIA-136, essentially collects signal strength measurements on all frequencies of the current band and identifies the strongest 24 channels. Then, starting with the strongest signal first, the MS determines if the channel is a DCCH. If so, the MS reads the F-BCCH to determine if the system is an autonomous system that is enabled in the MS. If so, the MS attempts a test registration. If the test registration is accepted, the MS displays the alphanumeric name of the autonomous system to the user and allows the user to initiate service acquisition on the autonomous system.
A problem with this possible solution is that the user is required to constantly monitor the MS to see if a reselection to the public system has occurred. This can become disruptive and annoying to the user. The user must also manually invoke the New PSID/RSID Search procedure, then wait for the MS to display the accept/reject message. This can become time consuming and also annoying to the user. The user's annoyance is further compounded by the expectation that the MS will stay registered with the autonomous system while the MS is in the autonomous system's coverage area.
Another possible solution is for the autonomous system to choose DTC channels of the public system for use as DCCH channels of the autonomous system that have a lower probability of being in use. Although this possible solution may help to reduce occurrences of interference, there is no guarantee that a channel will not be used by the public system. In fact, in very high traffic areas, it is possible for all DTC channels of a public system to be in use, effectively shutting down the autonomous system.